1. Field of the Invention
This invention relates to methods and apparatuses for liquefying natural gas. In another aspect, the invention concerns a liquefied natural gas (LNG) facility employing a system for separating accumulated incondensable components from one or more refrigeration cycles in an LNG facility.
2. Description of Related Art
Cryogenic liquefaction is commonly used to convert natural gas into a more convenient form for transportation and/or storage. Because liquefying natural gas greatly reduces its specific volume, large quantities of natural gas can be economically transported and/or stored in liquefied form.
Transporting natural gas in its liquefied form can effectively link a natural gas source with a distant market when the source and market are not connected by a pipeline. This situation commonly arises when the source of natural gas and the market for the natural gas are separated by large bodies of water. In such cases, liquefied natural gas (LNG) can be transported from the source to the market using specially designed ocean-going LNG tankers.
Storing natural gas in its liquefied form can help balance out periodic fluctuations in natural gas supply and demand. In particular, LNG can be “stockpiled” for use when natural gas demand is low and/or supply is high. As a result, future demand peaks can be met with LNG from storage, which can be vaporized as demand requires.
Several methods exist for liquefying natural gas. Some methods produce a pressurized LNG (PLNG) product that is useful, but requires expensive pressure-containing vessels for storage and transportation. Other methods produce an LNG product having a pressure at or near atmospheric pressure. In general, these non-pressurized LNG production methods involve cooling a natural gas stream via indirect heat exchange with one or more refrigerants and then expanding the cooled natural gas stream to near atmospheric pressure. In addition, most LNG facilities employ one or more systems to remove contaminants (e.g., water, acid gases, nitrogen, and ethane and heavier components) from the natural gas stream at different points during the liquefaction process.
Typically, LNG facilities employ one or more refrigeration cycles to cool the incoming natural gas stream by first condensing a stream of refrigerant and then contacting the vaporizing the refrigerant with the natural gas via direct or indirect heat exchange in order to reduce the temperature of the natural gas below its liquefaction point. Over time, one or more relatively incondensable components (e.g., air, nitrogen, helium, hydrogen, or argon) can build up in the refrigerant. Increased concentration of incondensable materials is highly undesirable because, for example, these relatively higher vapor pressure components don't condense at the operating conditions of the refrigeration cycle, thereby effectively diminishing the refrigeration capacity (i.e., duty) of the contaminated refrigeration cycle.
While the build up of incondensable materials in a closed-loop refrigeration cycle can occur, the problem is more pronounced in open-loop cycles, which employ a portion of the natural gas feed stream as the refrigerant. Minor changes in the feed gas composition can create substantial process disturbances and drastic swings in the composition of the natural gas feed stream can result in significant operational upsets, ultimately reducing the production of on-spec LNG produced from the facility for a certain period of time.